Field of the Invention
The invention relates to a process for cropping the head of self-luminous rolled material, especially metal strips, in which process the width of the head is measured, in advance of the set of shears, along a measuring line which runs at right angles to the direction in which the rolled material runs, and the shears are controlled in accordance with this measured value.
When rolling wide strips, a roughed strip is initially produced, from a slab, for instance in a reversing stand. Before processing this roughed strip further, it is necessary to determine the width of its head, because the head has not infrequently been widened during the roughing rolling process and exceeds the initial pass-width in the finishing stand. In order to enable a roughed strip, in this condition, to be processed further, the widened head must be severed (cropped) from the remainder of the rolled material.
An unsymmetrically shaped head can also lead to difficulties during the further processing. Thus, for example, the strip can come to be skewed during the pass through the first finishing stand. Should the head of the roughed strip be markedly tapered, rolling in the finishing stand produces a strip which, over its leading portion, does not satisfy the width requirement. For these reasons, the interests of the rolling mill operator are also served by cutting off the head portion, which is not to be used, prior to the finishing rolling operations.
In both cases, however, the portion which is to be cut off, as head-scrap, should be kept as small as possible.
In order to avoid trimming either too little or too much from the head, it is necessary to determine the width of the rolled material and the shape of its head. This is conventionally effected by means of measuring instruments which function in a contact-free manner.
Methods for performing dimensional measurements on rolled material, in a contact-free manner, are known to persons skilled in the art. In one method, which is used in practice, the rolled material is back-illuminated by means of a source of parallel light, which source is located beneath the roller table, and the optical system of a known diode-type line camera generates an image of the rolled material on the camera diodes (German Offenlegungsschrift No. 2,140, 939). In a camera which is suitable for carrying out this method, the diodes are charged by the incident radiation. The state of charge of the diodes is checked, in the course of which checking process they are discharged, so that they are again available for a new charging process. When a presettable charging time has elapsed, the state of charge is checked anew. The frequency at which the diodes are checked with respect to their charge is called the "sampling frequency."
In this known method for performing dimensional measurements on rolled material, in a contact-free manner, it is disadvantageous, nevertheless, that it is necessary to locate the illuminating lamps beneath the rolled material, between the rollers of the roller table. In particular, apart from the fact that the lamps are exposed to considerable amounts of thermal radiation, it is also impossible to prevent them from being damaged and contaminated by the scale which falls onto them from the rolled material.
In order to avoid these disadvantages, proposals have also already been made to utilise the radiation emitted by the rolled material itself for the width-measurement procedure. In a modern rolling mill, steels of different grades are processed into strip material. Since different grades must be rolled at different rolling temperatures, the result is that the intensities of the radiation emitted by the rolled material itself are subject to marked differences. In order to be able to perform optimum width-measurements, the exposure-adjustment system of the diode type line camera must be adjusted to suit the radiation from the rolled material which is running through the rolling train at the time in question. Additional difficulties result from the fact that the intensity of the radiation from the rolled material to be measured can decrease considerably from the middle of the material, towards the edges, so that the brightness-contrasts prevailing at the edges of the strip are frequently insufficient to enable reliable information to be obtained as to whether a photodiode in the edge-region is being disposed or not.
In order to locate the positions of the edges of the rolled material in an error-free manner, a method for measuring the width of self-luminous rolled material is known (German Offenlegungsschrift No. 2,516,756), this method being carried out with the aid of a diode-type line camera. In this method, the intensity of the radiation is measured by means of the line-camera diodes, and these measured values are conditioned for processing in a control system, the brightest point being used as the instantaneous brightness value for the control loop. The control system then adjusts the camera as a function of this conditioned measurement value. This method of adjusting the camera exposure-setting has the disadvantage that the exposure with respect to the edges of the strip, which are to be detected, is inadequate. It is accordingly necessary to provide a complicated additional device, in order to determine the abrupt change in brightness at the edges of the strip. To carry out this determination, the maximum abrupt brightness-change which is defined by the edge of the strip is initially determined in a learning procedure. This value is stored in a peak-value memory and is compared with the largest abrupt brightness-change occurring during the next measurement. Only when a corresponding abrupt brightness-change is again determined, is a trigger-pulse transmitted to a counter, which then outputs a measured value for the position of the edge of the rolled material. If, however, no abrupt brightness-change of equal magnitude is determined during the next measurement, no trigger-pulse is output to the counter, so that no measured value for the width is therefore obtained. On account of this learning procedure which, under some conditions, is constantly repeated, it is impossible to obtain measured values defining the position of the edges of the strip of rolled material from the very first measurement performed by the diode-type line camera. This disadvantage is of great importance, especially when the requirement involves the measurement of the shape of the head of the rolled material as it is running in. If, in fact, it is not possible to determine the width of the rolled material from the tip of its head, it is impossible to minimise the proportion of scrap material during the operation of cropping the head.
In a known process, the cropping operation is effected automatically, in accordance with the width of the strip, this width having been measured previously (German Offenlegungsschrift No. 2,011,276). The width of the rolled material which is running through is continuously measured at two points, these points being located at a fixed distance one from the other. As soon as the same width is determined at the two measurement points, a signal for initiating the cropping cut is transmitted to the set of shears. A process of this type is neither optimum nor, moreover, is it entirely reliable, since, depending on the shape of the head, it may be unnecessary, for the further processing, to trim off the entire head. Moreover, headshapes are conceivable in the case of which identical widths are determined at the preset measurement points, although a cut located in this region does not adequately render the beginning of the rolled material suitable for the further processing.